Glycol solvent such as ethylene and polyethylene glycol solvents are used in a number of important industrial applications, for example, dehydration of natural gas, production of anhydrous ethanol and other water soluble solvents and in separation of aromatic hydrocarbons from petroleum fractions. Examples of glycol solvents are tetraethylene glycol, triethylene glycol, diethylene glycol and ethylene glycol. Such glycols usually contain between about 0% to 5% by weight of water.
The use of glycols for dehydration of gases and, particularly, natural gas is disclosed in some detail in Chapter 11 of Gas Purification, Fourth Edition, Kohl and Riesenfeld, 1985. The principal glycols used in gas dehydration are diethylene glycol and triethylene glycol. As stated in the foregoing publication, the factors leading to the use of glycols are their unusual hygroscopicity, their excellent stability to heat and chemical decomposition, and their low vapor pressure. The dehydration process involves the use of a counter current system wherein the glycol stream containing from about 1% to 5% water is contacted with an upward flow of gas to be dehydrated, generally in the temperature range of about 85.degree. to 105.degree. F. After the dehydration process, the diluted glycol stream is regenerated or reconcentrated by distilling out the water absorbed, at atmospheric pressure at a reboiler temperature that does not exceed 400.degree. F.
The glycols as indicated above have also been used in the production of anhydrous ethanol and other water soluble type compositions to remove water. Ethanol when derived from a corn base as an additive to gasoline is initially formed as a solution of up to about 50% water. For use as a gasoline additive the water must be removed, by fractional and extractive distillation with a water solvent, such as one of the aforementioned glycols.
Further, the separation of aromatic hydrocarbons, such as the BTEX components (benzene, toluene, ethyl benzene and xylene), from petroleum fractions by liquid-liquid extraction frequently use as solvents glycols and, in particular, diethylene glycol (DEG) and triethylene glycol (TEG). A brief review of this process and improvements is available in Chapter 18.2. Handbook of Solvent Extraction, John Wiley & Sons, 1983.